Having succeeded in developing styrene-based polymers with high syndiotacticity, the research group of the present inventors further developed syndiotactic styrene-based copolymers through copolymerization of the styrene-based monomer as used in producing the syndiotactic styrene-based polymers, and other components (see Japanese Patent Application Laid-Open Nos. 62-104818 and 63-241009). However, though those syndiotactic styrene-based polymers and copolymers had good heat resistance and good chemical resistance, their stiffness and elongation were poor and their miscibility with other resins was also poor. Therefore, their use was inevitably limited. To improve their stiffness and elongation and even their miscibility with other resins, proposed were graft copolymers as produced through graft-copolymerization of a high-molecular polymer having double bonds in its side chains, and a styrene-based monomer; and block copolymers as produced through block-copolymerization of a macro-monomer terminated with a polymerizable active vinyl group, and a styrene-based monomer (see Japanese Patent Application Laid-Open Nos. 5-247147 and 5-295056).
However, those styrene-based graft copolymers and block copolymers as concretely illustrated in the laid-open patent specifications are problematic in their properties. Specifically, in the olefin-based segments to constitute them, the reactive groups exist at random. Therefore, when the olefin-based segments are copolymerized with a styrene-based monomer at the reactive groups therein, they are crosslinked to interfere with the effective formation of graft copolymers. At present, no styrene-based graft or block copolymers are obtained having satisfactory stiffness and even miscibility with other resins.
In order to improve their stiffness, the conventional styrene-based graft or block copolymers are blended with a rubber component such as hydrogenated styrene-butadiene-styrene block (SEBS) copolymer or the like. However, such is problematic in that the rubber component is expensive, that it indispensably requires the additional step of blending the copolymer with such an expensive rubber component, and that the addition of the amorphous component inevitably worsens the intrinsic characteristics (heat resistance, rapid crystallizability) of SPS (syndiotactic polystyrene). Accordingly, desired are techniques for producing syndiotactic styrene-based polymers or copolymers with high stiffness and even good miscibility with other resins.